When a tractor is used for towing a baler, the PTO shaft of the tractor is coupled by way of a flywheel of the baler to a gearbox that drives the various mechanisms used in the baler to pick-up the crop from a field, to form the crop into bales and to wrap twine around the bales. A one-way clutch is arranged between the PTO shaft and the flywheel to allow the flywheel to rotate faster than the PTO shaft.
A large flywheel is required in a baler, especially in large square balers, to overcome peak loads encountered by the baler gearbox and this creates a problem when starting the baler. The PTO shaft when driven by the tractor engine at the speed at which it needs to turn to drive the baler cannot provide the high torque needed to bring the flywheel up to its steady speed. As a result, when the baler is hitched to the tractor and its flywheel is connected to the PTO shaft, the tractor engine stalls or a safety mechanism declutches the PTO from the tractor engine on account of the high load on the PTO shaft when the baler is started.
A solution to this problem proposed in EP1974601 is to use a hydraulic motor to supplement the torque provided by the PTO shaft when starting the baler flywheel. This solution is not entirely satisfactory because it requires modification of the baler and it also calls for a source of hydraulic fluid under pressure.